1. Field of the Invention
This invention relates to a process for separating solid wax particles from a slurry comprising said wax particles and a hydrocarbon oil. More particularly, this process relates to an improved process for filtering solid wax particles from a mixture of at least partially dewaxed oil and solvent wherein the improvement resides in using a needled-felt filter cloth fabricated from fibers fusible by means of an open flame and having a singed surface on which the wax is collected.
2. Description of the Prior Art
Waxes are defined as animal, vegetable or mineral, depending on their source or origin. In their natural state, most of these waxes exist in solution in waxy oils. In order to separate the wax from the waxy oil the oil is chilled, usually in the presence of a solvent, to precipitate the wax therefrom. The solvent functions both to reduce the solubility of the wax in the oil and to reduce the viscosity of the resulting dewaxed oil thereby greatly facilitating filtration of the waxy slurry to separate the precipitated wax from the solution of dewaxed oil and solvent. Various filtration means may and have been employed to separate the wax from the oil and solvent such as the use of plate and frame presses, shell and leaf filters, cartridge filters and rotary-drum filters. Continuous rotary-drum filters are well known and used in the petroleum industry for wax filtration, particularly for filtering wax from dewaxed lube oil fractions. A typical rotary-drum vacuum filter comprises a horizontal, cylindrical drum, the lower portion of which is immersed in a trough containing the wax slurry, a cloth filter medium or filter cloth covering the horizontal surface of the drum, means for applying both vacuum and pressure thereto and means for washing and removing wax cake deposited on the cloth as the drum continuously rotates around its horizontal axis. In these filters the drum is divided into compartments or sections, each section being connected to a rotary (trunnion) valve and then to a discharge head. The wax slurry is fed into a filter trough and, as the drum rotates, the faces of the sections pass successively through the slurry. In a vacuum drum filter a vacuum is applied to the sections as they pass through the slurry, thereby drawing oily filtrate through the filter cloth and depositing wax thereon in the form of a cake. As the cake leaves the slurry it contains oily filtrate which is removed therefrom by the continued application of vacuum, along with wash solvent which is evenly distributed or sprayed on the surface of the cake. Finally, the washed wax cake is removed from the surface of the filter cloth by a scraper which is assisted by means of blow gas applied to each section of the drum as it rotates and reaches the scraper. In a pressure filter, the dewaxing solvent contains an autorefrigerant, which, by virtue of its relatively high vapor pressure, is sufficient to apply a pressure differential across the filter surface of the drum, thereby eliminating the need for applying a vacuum thereto.
Little attention has been paid to the type of filter cloth employed for wax filtration. For years, conventional woven filter cloths have been employed fabricated from natural or synthetic yarns. As the cycle of wax deposition and removal from the filter cloth continues, the throughput of the dewaxed oil and solvent steadily decreases due to clogging of the cloth with wax particles which is called blinding of the filter. After the filter has been in operation for a period of time the blinding continues to a point where the filter cloth has to be washed with a solvent such as hot kerosene to dissolve and wash away the wax particles trapped therein which have been causing the reduction in the filter rate. In some applications, it is very possible for this wash cycle to occupy four hours of a 24 hour day. Thus, not only does the blinding of the filter cloth result in a loss of throughput, but it also requires frequent washings and when the filter cloth is being washed the filter is down from a production standpoint in that it cannot be used to filter wax from the slurry.
Therefore, any improvement to wax filtration processes employing a cloth filtration medium or filter cloth which would reduce blinding of the filter cloth would be a substantial improvement to the art.